Common knowledge but recordings (vinyl, radio) of the last 70 years have little musical content above 20Khz. Loudspeakers have top end response of 20Khz as well. However, when CD players appeared in 1983 it was found that the digital circuitry produced quantization noise at 44khz and beyond. As a result "brick wall" filters were put in CD players to block everything above 20KHz to keep the audio "clean" and distortion free. Problem solved.
However, this sudden focus on noise revealed that house wiring carries a lot more than just 60HZ. Along with AM/FM radio, high frequency noise was found to be present well beyond 100Mhz. This is created by PCs, fans, light dimmers, routers, wifi, appliances...you name it. All it takes is one noisy unit to dump AC trash and other artifacts into the entire house's wiring. Even worse, everyone in the neighborhood is suspect because they tap into the same overhead powerline...cross-polluting each other. This is an issue because an audio amplifier/preamp can't distinguish between line noise and music and will try to amplify everything. Even if RFI noise is not audible it wastes power, increases harmonic distortion and is contrary to the concept of reproducing the purest signal possible. Its like mixing an expensive cocktail with top-shelf ingredients and using dirty water. As a result surge suppressors were developed with two features: EMI/RFI filtering....and voltage spike protection.
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STAGE 1 FILTER
This schematic below evolved over the decades and is used worldwide for AC filtering. In reality its a simple high-current, low-pass crossover designed for 60HZ. Important to realize that only so much filtering can be added without causing insertion loss and distorting the 60HZ sine wave.
- Special "X" and "Y" type capacitors are used. One X cap goes between the rails (called Differential mode) while two Y caps go from rail to ground (called Common mode). An X type burns through (open) when blown while the Y keeps drawing current when blown to maintain grounding. These caps filter out EMI/RFI.
- Toroidal chokes and wire-wound coils are used inline with each rail to filter out high frequencies.
- Three MOVs are typically used. These increase resistance as voltage rises and clamp down on spikes.
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ISOBAR by TRIPPLITE
Tripplite has been making the Isobar surge suppressor since the early 1990s. While most power-strip surge suppressors of the period (and today) are cheap plastic with no protection...the rugged metal Isobar contains "toroidal chokes, ferrite rod-core inductors, HF/VHF capacitors and multiple layers of metal oxide varistors that remove EMI/RFI interference, even noise generated by other pieces of connected equipment." These affordable little units made it into many audio systems and have been a guilt-free staple in my mine for years.
BELOW: a recent Isobar Ultra 6 is at the top and an older 1992 Isobar on the bottom. The angle of the photo slightly exaggerates the size of the old one.
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The NEW Isobar adds two ferrite-rod inductors and several more MOVs. However the levels of bank filtering are no longer marked on the front. Tripplite states that each bank of the new version is isolated but only states a single -80db spec of EMI/RFI filtering. This suggests that each bank is not isolated from the others as in the past.
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Tripplite was way ahead of the pack in the 1990s when it released the Isobar. The use of chokes and coils in a budget-level unit was unusual for the period and provided a unique product for the audio community. While passive filtering is a step below a true line conditioner which uses a transformer to supply constant voltage...it provides genuine noise filtering and line protection.
The 1992 Isobar contains fewer filtering components than the modern version. And its method of progressively stronger filtering to each bank wasn't flexible if you wanted maximum filtering for ALL audio components.
Interestingly the modern version appears to forgo individual EMI filtering per bank in favor of group filtering. In my opinion the best Isobar would be would be identical EMI/RFI filtering on all banks, with isolation between each bank. This could be accomplished by Tripplite with some wiring changes without much fuss.
NOTE: The concept of using chokes and capacitors to filter out AC line noise is decades old and proven. Since people who understand the concepts are likely to buy the better quality units...advertising should cater to them. This means companies should openly state the components used inside and explain the schematic they use. This is the only way for the informed consumer to compare products and their effectiveness.
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